The Fischer-Tropsch synthesis process (FTS) that is used to produce hydrocarbons has been known for many years now and is described, for example, in detail in Ullmanns Encyclopedia of Technical Chemistry, vol. 4, 14th edition, pp. 329ff., Verlag Chemie, Weinheim (1977). In this method, raw gas, usually involving synthesis gas from coal gasification and composed primarily of carbon monoxide (CO) and hydrogen (H2) after partial oxidation is converted into liquid hydrocarbons by heterogeneous catalysis. Aside from the remaining residual FTS gas, what is generated in particular are liquid products, in particular, aliphatic compounds and olefins. The FTS process has once again gained in importance in light of the fact that the cost of refined petroleum products has been increasing at a steady rate in recent years.
In plants operated today that have a Fischer-Tropsch synthesizer (FTS unit), the goal in terms of achieving an optimal yield is a gas composition having an H2:CO molar ratio of approximately 2:1 when using predominantly iron-based catalysts. In order to improve the utilization of the CO and H2 components contained in the input gas, a portion of the FTS product gas is compressed and recycled back into the input gas stream. The recycle ratio is selected here such that up to two times the quantity of the input gas is recirculated. The recycle ratio is limited by the fact that the inert gas fraction, such as nitrogen, argon, and carbon dioxide (CO2), is successively increased as the recycling is repeated, and due to this factor no additional recycling is economically useful. Specifically, the CO2 component increases disproportionately in the process gas since a portion of the CO input is converted to CO2. This limits the yield of the input raw gas to recycle ratios of less than 2.5, where the remaining residual gas still containing CO and H2 is discharged from the process.
The molar ratio H2:CO of a synthesis gas obtained from coal gasification is approximately 1:3, and is thus fundamentally unsuited for direct feed to a Fischer-Tropsch synthesis reactor. In current plant designs, a partial stream of the raw gas is therefore processed before being fed to the FTS unit, where pretreatment of the process gas is essentially composed of a desulfurization stage and a CO converter. A differentiation is made here between sulfur-containing conversion (sour shift) and desulfurized conversion (sweet shift). The H2:CO molar ratio in the process gas is adjusted in both cases by reacting part of the contained CO with steam to produce H2 and CO2.
Since a relatively high energy requirement is incurred in the process, in part due to the required compressor capacity, a partial stream of the residual FTS gas is fed to an energy recovery stage to improve the energy balance. One or more gas turbines are used here in combination with one or more generators to generate electric current that is in turn supplied to the plant when in operation.